The invention relates to a multicast service providing message delivery to selected subscribers in a mobile telecommunications network.
In the digital GSM mobile communications system (Global System for Mobile communications), in addition to speech and data connections a short message service is implemented. Short messages are transmitted along control and signaling channels with other signaling of the network. Short message services can be divided into point-to-point and cell broadcast services. Point-to-point short messages are either mobile originating or mobile terminating. Cell broadcast short messages are transmitted from base stations and are intended for all mobile stations in the coverage area of the transmission. Examples of cell broadcast services are traffic and weather information delivery.
FIG. 1 of the attached drawing shows a simplified block diagram of the GSM mobile communications system. The mobile station MS is connected via a radio path to a base transceiver station BTS, in FIG. 1, to the base station BTS1. A base station sub-system BSS consists of a base station controller BSC and, the base stations BTS controlled by it. A mobile services switching center MSC usually controls several base station controllers BSC and is connected to other mobile services switching centers and a GMSC (Gateway Mobile Services Switching Center). Via the GMSC, the GSM network is connected to other networks, such as the PSTN (Public Service Telephone Network), another mobile communication network PLMN, the ISDN network, the intelligent network IN, or the short message service center SMSC. The operation of the entire GSM system is monitored by the operation and maintenance center OMC.
FIG. 2 of the attached drawing illustrates three cells C1, C2, and C3, which offer cell broadcast messages to all the mobile stations MS located in these cells, including the one marked MS1. It would also be possible to transmit cell broadcast messages only from some cells, for example, from cells C1 and C2. The cell broadcast messages are created by a cell broadcast entity CBE, a news agency, for example, and they are transmitted to mobile stations MS via an appropriate mobile communications network. The CBE transfers the cell broadcast message to a cell broadcast center CBC connected with the BSC in the GSM network, in this example. The cell broadcast center CBC controls the message transmission on the cell broadcast channels of individual BTSs, and transmits via the BSC the messages to each BTS addressed. The cell broadcast is transmitted across the air interface in the cell broadcast channel BCCH of the BTS or in paging channels PCH. Cell broadcast is intended for all receiving mobile stations, therefore it is not addressed nor encrypted. For a more detailed description of cell broadcast, a reference is made to the GSM-specification GSM 03.41 xe2x80x9cEuropean digital cellular telecommunication system (Phase 2); Technical realization of Short Message Service Cell Broadcast (SMSCB)xe2x80x9d.
In order to implement the point-to-point short message service, the GSM network is connected to the short message service center SMSC. The SMSC forms a connection between a particular short message network and the GSM network by transferring the shotmessage from outside the GSM network to a mobile subscriber and from one mobile subscriber to another or to some other unit outside the GSM network that is able to receive short messages. A mobile terminating short message is routed from the short message network via the short message service center SMSC to the MSC of the GSM network and from there via the appropriate BSC and BTS to the mobile station MS, i.e. via BTS1 to MS1 in FIGS. 1 and 2. Accordingly, a mobile originating short message is transmitted from the MS via BTS and BSC to the MSC which transfers the short message from the GSM network to the SMSC, in order to be transmitted to the appropriate short message network. Short messages are transferred transparently in the network between the short message service center SMSC and the mobile station MS.
The confidentiality of the transmission is sometimes arranged with ciphering, for example. FIG. 3 of the attached drawing shows a block diagram of a known symmetric cipher algorithm which can be used to protect transmission. The symmetric ciphering algorithm is based on a secret key shared between the communicating parties. At party A""s end the message M to be sent to party B is encrypted in box E in FIG. 3 with the shared secret key K. The message is sent over a transmission route as the encrypted cipher text C, which party B can decrypt in box D shown in FIG. 3 with the same secret key K. Through decryption party B gets the original message M.
An intruder eavesdropping on the transmission needs to know the secret key K in order to be able to read and understand the transmitted cipher text C. The encryption and decryption of the symmetric algorithm can be expressed by the equations:
C=EK(M)
M=DK(C),
where C is the cipher text, M is the message in plain text, EK is the encryption with key K, and DK is the decryption with key K. Presently, ciphering is not applied to the short messages routed through the GSM network except in the air interface.
The problem with known short message services is that messages are either transmitted to all mobile stations in the coverage area of the transmission (cell broadcast) or with dedicated transmission to one addressed mobile station at a time (point-to-point). Neither of these transmission methods is useful for transmitting messages to a selected group of mobile stations, as prior art cell broadcast cannot be directed only to some of the mobile stations, and transmitting the same message with several point-to-point short messages to a group of mobile stations causes too much signaling in the network and requires a relatively long time to provide all the recipients with the message.
The object of this invention is to provide multicast transmission to a selected group of mobile stations in a way that minimizes the signaling in the network.
The invention is based on the idea that a broadcast message is used for multicast transmission and a token is needed at the mobile terminal to recover the information sent in this broadcast message intended for multicast. The token is delivered to selected mobile terminals in a point-to-point short message, for example. In the primary embodiment of the invention, the cell broadcast messages intended for multicast are encrypted and the keys for decryption are delivered only to those mobile terminals entitled to receive the multicast messages. The mobile stations store the delivered token, which is in this case the cryptographic key, in their memory, in the SIM card, for example. In the secondary embodiment of the invention, selected mobile terminals are enabled to receive and recover the multicast messages. The token delivered is a command removing the restriction in order to recover the multicast transmission at the mobile terminal. For example, the command might be an instruction to start to listen to the multicast or an instruction to tune in to a certain channel.
The advantage of the method according to the invention is that it provides a way to implement a short message service to a selected group of mobile terminals on a subscription basis.
Another advantage of the method according to the invention is that it uses network capacity sparingly, since the information can be delivered to several mobile terminals with only one short message.
The advantage of the arrangement according to the invention is that the token delivery and the recovery of the multicast messages is transparent for the mobile subscriber.